25 MHz intravascular ultrasonic catheters and instrumentation available from the field of nondestructive materials testing are proposed to allow high resolution, in situ imaging and tissue characterization of atherosclerotic plaque. The use of frequencies higher than usually available from current medical instrumentation is necessary to yield lateral resolution greater than 0.5 mm from the samll aperture transducer, and also has the additional benefit of increased backscattering from smaller inhomogeneities. In addition ultrasonic rf tissue characterization techniques using current medical bandwidths can be used with the increased bandwidth available from the intravascular approach to provide estimates of plaque composition. Ultrasound also provides the ability to examine structures deeper than the intimal lining so as to demonstrate mural atherosclerotic involvement or the attachment of mural thrombi. In conjunction with a commercial transducer manufacturer and based on preliminary success, ultrasonic catheter transducers will be developed by mounting the small transducer in standard medical catheter delivery systems. A high frequency ultrasonic instrument with the capabilities of imaging, signal averaging, spectral analysis and data storage will be purchased. Data acquired from interesting plaque formations will be stored on the imager's floppy diskette for down loading to a general purpose minicomputer. Tissue characterization software already developed for a currently funded project will be modified and used to compute ultrasonic attenuation, mean backscatterer size, and integrated backscattered power. Experimental verification of the ultrasonic system's imaging and tissue characterization capabilities will be obtained from both animal and human studies. Three groups of rabbits (normal, midstage, and endstage atherosclerosis) will be examined during the first year. Similarities between the ultrasonic images and sliced aortic cross-sections will be noted. Atherosclerotic lesions removed from the aortas will be assayed for calcium, collagen and lipid content. Multiparameter statistical correlations will be examined between the pathological data and computed tissue characterization parameters. In the second year human studies will be performed to demonstrate correlations between intravascular ultrasonic imaging and angiography. In addition ultrasonic tissued characterization of lesions will be performed to demonstrate similar statistical patterns seen from rabbit data.